A.T. Robertson Commentary Philemon 1:9

"yet for love`s sake I rather beseech, being such a one as Paul the aged, and now a prisoner also of Christ Jesus:" — Philemon 1:9 (ASV)

Paul the aged (Παυλος πρεσβυτης). Paul is called νεανιας (a young man) at the stoning of Stephen (Acts 7:58). He was perhaps a bit under sixty now. Hippocrates calls a man πρεσβυτης from 49 to 56 and γερων after that. The papyri use πρεσβυτης for old man as in Lu 1:18 of Zacharias and in Tit 2:2. But in Eph 6:20 Paul says πρεσβευω εν αλυσε (I am an ambassador in a chain). Hence Lightfoot holds that here πρεσβυτης = πρεσβευτης because of common confusion by the scribes between υ and ευ. In the LXX four times the two words are used interchangeably. There is some confusion also in the papyri and the inscriptions. Undoubtedly ambassador (πρεσβευτης) is possible here as in Eph 6:20 (πρεσβευω) though there is no real reason why Paul should not term himself properly "Paul the aged."